This is quite noteworthy. Following a jailbreakhack which purports to make your unlocked AT&T iPhone 5 (model A1428) work on T-Mobile’s LTE by way of loading a custom carrier firmware, a new report claims the Deutsche Telekom-owned wireless carrier itself will be sending out a carrier update designed to officially enable T-Mobile’s LTE and Visual Voicemail, among other features, on unlocked GSM devices.

The update is scheduled for delivery beginning April 5, a week ahead of T-Mobile’s official April 12 roll-out of its LTE-ready iPhone 5. The patch is good news for AT&T customers looking to jump ship as they won’t be required to buy a newly tweaked iPhone 5 model from T-Mobile in order to enjoy T-Mobile’s 4G LTE speeds. There’s more to know so go past the fold for full details…

TmoNews notes that this is an Apple update so it should hit iTunes or all iPhone devices around the same time. The publication also posted a credible-looking photo of an internal T-Mobile document, seen above.

The doc reads:

The T-Mobile Carrier Update is a minor iOS software update that enables official iPhone support by T-Mobile.

When installed, the software update enables a handful of capabilities like Visual Voicemail, MMS Settings and Network/Device optimizations that customers do not have access to today.

On April 5, the software update will begin being pushed via OTA to all iPhone devices on the T-Mobile network with iOS 6.1.x or higher.

Again, come April 5 you should be able to take your unlocked AT&T iPhone 5, apply the patch and immediately start enjoying T-Mobile’s LTE. On the other hand, you will not be able to run a patched device on T-Mobile’s AWS Band 4 frequencies used for its 3G network because iPhone devices sold to date lack hardware support for AWS Band 4.

In other words, the AT&T iPhone 5 variant patched with Apple’s carrier update will fully support T-Mobile’s LTE, but won’t be able to fall back to the carrier’s 3G network outside of LTE coverage. Instead, these devices will revert to the sluggish T-Mobile EDGE network on Band 2.

This is precisely why Apple had to create a factory tweaked iPhone 5 version for T-Mobile. Despite carrying the same model number A1428 as the existing AT&T iPhone 5, the tweaked variant features a slightly re-jigged hardware for full compatibility with T-Mobile’s 4G LTE and 3G DC-HSDPA network.

Apple previously confirmed to Engadget it couldn’t just roll out a software update to the existing A1428 devices as the necessary firmware tweak has to be applied at the factory.

“Apple affirmed to us that it’s not something that can be enabled via a simple software update for A1428 iPhone units already in circulation,” Engadget claimed last week.

Apple also promised to phase out the existing AT&T A1428 iPhone 5 variant in favor of the factory tweaked A1428 model. Not only is it compatible with both T-Mobile’s 4G LTE and 3G DC-HSDPA, it actually supports all AT&T and T-Mobile frequencies.

Last week, AT&T confirmed plans to roll out HD Voice on its network “later this year.” HD Voice, or Wideband Audio, is a network feature T-Mobile will support first among major U.S. telcos when its iPhone 5 launches on April 12.